The use of nuclear magnetism in well logging in making direct measurements on the hydrogen content in a subterranean formation and not on the rock matrix has been described by R. J. S. Brown and G. W. Gamson in "Nuclear Magnetism Logging", Petroleum Transactions, AIME, Vol. 219, 1960, pp. 199-207. Briefly, many atomic nuclei possess magnetic moments and spins. If a magnetic field is applied, the magnetic nuclei tend to align in the direction of the applied magnetic field. There is a net magnetization, or polarization, which is directly proportional to the strength of the applied magnetic field. When the magnetic field is changed, a new equilibrium value of proton polarization is not immediately established, but requires an amount of time which depends on the nature of the hydrogen content. This process of approaching equilibrium is called "relaxation". Brown and Gamson describe a basic nuclear magnetism logging (NML) tool in which current is passed through a coil to produce a magnetic field in the borehole and in the formation surrounding the borehole at a right angle to the earth's magnetic field. This field, called the "polarizing field", serves to align protons in the formation fluids. When adequate polarization has been established, the current is removed and the polarization precesses in the earth's magnetic field, thereby inducing a damped sinusoidal signal in the same coil originally used to induce the polarization. At the moment precessing begins, such signal is directly proportional to the total number of protons per unit rock volume in the water, oil and gas in the vicinity of the borehole. During the signals-receiving time, the coil is connected to an amplifier and the signal is sent over a logging cable to the surface of the earth. This signal is recorded as an indication of the amount of hydrogen in the formation fluids surrounding the borehole. In addition to this NML tool described by Brown and Gamson, similar NML tools are fully described in U.S. Pat. No. 3,667,035 to Slichter (1972) and in a paper entitled "An Improved Nuclear Magnetism Logging System And Its Application To Formation Evaluation", by Herrick, Couturie and Best, presented at the 1979 Annual Meeting of the Society of Petroleum Engineers of AIME (SPE 8361).
NML tools as described above require a polarization pulse duration of about five times the characteristic time constant for the expotential build-up of the induced polarization and is in the order of about two seconds. However, this polarization pulse duration can be reduced to the order of a few tens of milliseconds or even further to a few tens of microseconds by the technique of radio frequency (RF) pulse nuclear magnetic resonance (NMR). The average power required to generate a polarizing pulse is reduced due to the shorter time period, or for the same average power, the peak power, or energy, in the pulse can be increased. The magnitude of the NMR signal induced in the receiver coil due to the relaxation of the magnetic moment following the termination of the RF pulse is determined by the rate of change of the RF magnetic field produced by the decaying magnetism. This rate of change is determined by the static magnetic field that is present. Such RF pulse NMR logging tools have been described by Brown, Jackson and Koelle in "Western Gas Sands Project Los Alamos NRM Well Logging Tool Development", Los Alamos Scientific Laboratory Report LA-10374-PR (March 1985) and by Claw, Percival and Walters in U.K. Patent Application GB No. 2,141,236-A (Dec. 12, 1984). Both these references describe the generation of a static magnetic field that is significantly greater than the earth's magnetic field. This static magnetic field is generated by the use of a pair of magnets with like poles opposing. The result in a toroidal region of very small volume at some radial distance that is substantially homogenous. The magnitude of the magnetic field in this region is determined by the strength of the pair of magnets. Since the volume of formation material that is polarized within this homogeneous region is small, the resulting NMR signal is weak. The RF transmitter-receiver coil is located between the opposing pole faces of the pair of magnets. This NMR tool has an omnidirectional sensitivity in a plane perpendicular to the axis of the pair of magnets as defined by the toroidal region. Because of this omnidirectional sensitivity, the NMR tool must be centralized in the borehole with the resulting NMR measurement suffering from borehole fluid sensitivity to the RF polarizing pulse. It is, therefore, a specific feature of the present invention to provide an NMR logging tool that overcomes this shortcoming of measuring weak NMR signals from the formation in the presence of interfering, and sometimes dominant, borehole fluid RF signals.